1. Field
The instant invention relates to capacitance-type sensors for measuring changes in capacitance due to changes in compositional concentrations of an oil-water-gas or oil-water mixture, especially in a dynamic, that is, flowing, oil-water-gas system.
2. State of the Art
Sensors of various types have been used to measure the concentrations of water and gas in oil. It is often useful to know whether in undersea operations there is water leaking into the oil so that values can be periodically compared to determine changes in water content or in situations of ownership transfer or multiple mixture ownership where precise measurement of composition is required. Some such sensors have used the impedance of the mixture in order to measure changes in impedance and thus determine changes in concentrations. Impedance-type sensors, however, have to have the electroconductive member in contact with the fluid.
However, because of the erosion and corrosion characteristics of the flowing liquid, the use of metal films generally has not been satisfactory. Thus, a pair of exposed metal conductors on either interior lining of the bore of the sensor can be quickly eroded. Additionally, the variation in electrical impedance of the mixture components, independent of the true mixture composition, makes precise measurement impossible.
Capacitance-type sensors are also useful, however, the capacitance plates cannot be readily exposed to the moving fluid since the fluid components are relatively electrically conductive, thus shorting the capacitance plates. The capacitance plates, therefore, must be electrically isolated from the fluid and from one another. Any material between the capacitance plate and the fluid, of course, will also affect the overall capacitance measured by the device. Because of the high pressures involved, e.g. greater than 150 bar, and changes in temperature, any material used to separate the capacitance plates from the fluid must be strong, abrasion resistant, and have a low permittivity and low thermal coefficient of expansion.